The Very Cold Case of the Glacier

It just so happens that this deglaciation -- the more correct term for an expiring ice sheet -- is an ancient climatological whodunit. And therefore, on a recent afternoon, like a homicide detective trying to clear a very old case, Joerg Schaefer was placing a small chunk of quartz from Central Park in his sample bag.

What, then, killed the glacier, and how long did it take to die? "We don't really know," said Dr. Schaefer, a researcher at the Lamont-Doherty Earth Observatory at Columbia University. Though his methodical quest could seem of interest only to geology buffs, it may shed new light on the accelerating and persistently controversial phenomenon of global warming, which has chased glaciers into retreat across the planet and could bring New Orleans-style flooding to coastal cities around the world.

The New York region was once covered by a vast crystalline shield of frozen water, known as the Laurentide ice sheet. It carved the terrain of the metropolitan area, and as it melted, dumped so much transported rock, gravel, sand and sediment that it created parts of Long Island, Connecticut and New Jersey -- including the barrier islands at the coast. It also deposited such notable landforms as Battle Hill, in the Green-Wood Cemetery in Brooklyn.

"The rocks of New York City are a climate archive," Dr. Schaefer said. Most New Yorkers are unaware "that they are living in the middle of a glacial event park," he said, adding: "All they need do is open their eyes. By looking into the past, we can learn about the sensitivity of glaciers as climate indicators."

And so, on Long Island, in Manhattan, and at locations up the Hudson River toward Albany, Dr. Schaefer, 36, and his three-scientist team are measuring the retreat velocity of the last glacier. What makes this possible for the first time is a new age-measuring technique that has one simple, but seemingly unreasonable, requirement: the testing of relatively clean surfaces that have been undisturbed for 18,000 years.

Enter, Central Park.

Not to mention Marcus Garvey Park in Harlem; Inwood and Morningside Parks on the West Side of Manhattan; a pristine glacial expanse in Harriman State Park; and even a truck-size glacial boulder in Port Jefferson, N.Y. (a parking lot was built around it, given its size). "It's spectacular that in such an urban setting, there are these completely ancient features," Dr. Schaefer said.

In Central Park, for example, much of the visible bedrock was shaped by ice, and unmodified glacial features abound. They include striations (abrasion grooves that show the flow direction from northwest to southeast), glacial polish (caused when rock was buffed by sediment), chatter marks (gouges in bedrock made by glacier-dragged stones), and erratics (boulders stranded on bedrock by the glacier).

One of the most impressive glacial remnants in Central Park is Umpire Rock (so called thanks to its proximity south of the Heckscher Ballfields), to the east of West 62nd Street, by the pétanque court.

The feature is a rarity in that its deep grooves reveal a carved channel and glacial fissures that suggest "possible evidence of subglacial streams," Dr. Schaefer said.

"As you see the deep grooves, you can almost imagine these big boulders gouging out the bedrock," said Neil Calvanese, vice president for operations of the Central Park Conservancy, which manages the park under a contract with the city.

Dr. Schaefer's Central Park rock sampling has been conducted with the park's official blessing. As Mr. Calvanese said, "We don't encourage people whacking at our rocks." Through the years, the park has attracted research endeavors from astronomy to environmental science. In 2002, a new invertebrate, Nannarrup hoffmani, was discovered there.

Scientists have also compared the park's ginkgo trees to fossil ginkgos whose leaves were munched by the dinosaurs. The United States Geological Survey has monitored ground water in the park, and Lamont has also maintained a seismograph in the North Meadow.

But for the first time in the park, Dr. Schaefer is employing a new scientific tool called "cosmogenic dating," a name that describes not a New Age mate-matching service but rather a pioneering way of measuring the age of landforms.

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The key is to identify beryllium-10, an unstable isotope, or radionuclide, which forms in locations that have been struck by cosmic rays, including rock surfaces.

As glacial ice retreated, "it opened up the rock to cosmic rays," Dr. Schaefer said. "An isotope is created at the moment the cosmic rays strike the rock, and when the surface is exposed, the clock begins ticking." The unstable isotope formed in the rock has a half-life of 1.5 million years, a rate of radioactive decay that can be measured.

Beryllium-10 accumulates in quartz, which has veined much of New York City's bedrock, including the Manhattan schist that underlies Central Park. Precision in identifying glacially exposed quartz "is crucial in taking the samples," Dr. Schaefer said, not only to get the right data, but also because, at $500 to $2,000 per test, the geological team cannot afford to choose too many wrong outcrops.

Therefore, an unconventional but decidedly low-tech research tool was a 1782 British Headquarters map from the occupation of New York in the Revolutionary War. The team referenced its depiction of Manhattan's streams, lakes and landforms while roaming Central Park to identify undisturbed glacial outcrops.

Dr. Schaefer and his team chisel out small pieces of quartz, number them with red marker, digitally photograph them and fix their latitude and longitude with a global positioning unit. In the lab, the rock is pulverized and, in a complex process, beryllium-10 is isolated from contaminants, then measured with a mass spectrometer to determine how long ago it was exposed to cosmic rays.

"We can date the retreat of the glacier to within 500 years with prime samples," said Dr. Schaefer, who is a geochemist. He hopes to reconcile his glacial-dating techniques with the ages of Hudson River marine sediments and marsh sediments. And his team hopes that study of the British Headquarters map may yield clues about subglacial water channels and patterns of ice-sheet melting, which tended to dump erratics in north-south alignments.

Central Park may evidence some "boulder trains," lineups of glacial erratic boulders that could mark debris fields deposited by subterranean water channels or at the meeting points of "glacial lobes," fingers of the ice sheet that stretched southward, then melted northward.

One such alignment may begin with a large erratic perched 70 paces to the east of the Sheep Meadow Cafe at the meadow's northeast margin; to the south, another giant erratic marks the meadow's southeastern margin, roughly parallel to West 67th Street. Two other large erratics to the south may be part of this train.

However, "there are boulders in the park that seem to be obviously placed," Mr. Calvanese said. "Luckily, Olmsted didn't move the rock outcroppings," he said of Frederick Law Olmsted, who designed the park with Calvert Vaux. The last glacial maximum -- the greatest extent of the ice sheet -- was 18,000 years ago, and the Lamont team has been comparing its Central Park age measurements with samples taken by Dr. Schaefer in Switzerland, New Zealand, the Tibetan Plateau and the Sierra Nevadas. His team is still identifying new sampling locations and analyzing data so that its key findings can be submitted for peer review.

So far, though, the worldwide evidence indicates that "wherever we look, the glacier seems to have decided to retreat at the same time -- roughly 18,000 years ago," he said. "We have to figure out what global mechanism triggered this, if it is true."

Although it has been long believed that ice sheets took a long time to melt, glacial systems "may be much more quickly moving than we thought before, and they may react on pretty small climate changes in a very dramatic way," Dr. Schaefer said. "The indications are that the rate of collapse is faster than previously believed." Some scientists have theorized that the rapid melting of prehistoric glaciers could have triggered powerful climatic change. Eventually, as the last ice sheet melted, the planet entered the relatively warm, unusually stable interglacial era it currently enjoys.

If the last deglaciation happened rapidly, as Dr. Schaefer's research may indicate, it could mean that the current ice retreat -- seen from Peru to Tibet to Greenland -- could also switch from slow to abrupt. Some scientists are concerned that this could accelerate the ongoing rise in sea levels, and potentially add enough fresh water to the Atlantic to block the warm Gulf Stream, cooling Europe and perhaps the Northeast.

In other words, a modern counterpart of the 18,000-year-old global-warming event could trigger a new ice age. "Historically, interglacials are rare in the cold history of the planet," Dr. Schaefer said. "The system is perfect now for humans to be here, and it seems extremely fragile. If you push it out of stability, it could get dramatically worse."

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A version of this article appears in print on September 14, 2005, on Page B00001 of the National edition with the headline: The Very Cold Case of the Glacier; Using Central Park to Study Global Warming and Flooding. Order Reprints|Today's Paper|Subscribe